Low-pressure and nascent yields of stabilized Criegee intermediates CH 2 OO and CH 3 CHOO in ozonolysis of propene.

The yields of stabilized Criegee intermediates (sCIs), both CH 2 OO and CH 3 CHOO, produced from ozonolysis of propene at low pressures (7-16 Torr) were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO 2 ). The method of monitoring the consumption of SO 2 as a scavenger and the production of secondary formaldehyde (HCHO) allows characterization of the total sCI and the stabilized CH 2 OO yields at low pressure and in a short residence time. Both the total sCI and the stabilized CH 2 OO yields in the propene ozonolysis were found to decrease with decreasing pressure. By extrapolating the 7-16 Torr measurements to the zero-pressure limit, the nascent yield of the total sCIs was determined to be 25 ± 2%. The ranges of nascent yields of stabilized CH 2 OO and stabilized CH 3 CHOO were estimated to be 20-25% and 0-5%, respectively. The branching ratios of the stabilized and high-energy CH 2 OO* and CH 3 CHOO* were also determined.